Ultrasound, as a non-thermal food processing technology, is applied to bring positive effects in food processing, such as food preservation, improvement in mass transfer, the assistance of thermal treatments, the alteration of texture, and food analysis . Ultrasonic waves (also called supersonic waves) are sound waves in the frequency ranges of 20 kHz to 100 kHz . Ultrasound produces ‘cavitation’ in liquids, pressure variations in gas media, and liquid movement in solid media, respectively . It can be viewed as a form of high-frequency vibration that generates fluid mixing and shear forces on a micro scale .
The application of ultrasound in the food industry is divided into low energy and high energy approaches depending upon the sound power (W), sound intensity (W/m2
), or sound energy density (Ws/m3
) used. An ultrasound frequency higher than 100 kHz and intensity below 1 W/cm2
is preferred for low-energy applications that normally do not change the physical or chemical properties of the material through which they propagate 
. These are normally used for analytical applications, such as the determination of the physico-chemical properties of the materials, composition, ripeness, firmness, sugar content, and acidity of fruits and vegetables 
. Conversely, high-energy (power ultrasound) applications use frequencies between 20 and 100 kHz, and intensities that are higher, in the range of 10–1000 W/cm2
can alter the physicochemical properties or structure of a material 
. Today, it can be more effectively used for enzyme inactivation, the enhancement of drying and freezing processes, the extraction of essential oils, the control of crystallization processes, and the degassing of liquid foods 
3. Applications of Ultrasonic Waves in Food Processing
3.1. Microbial Inactivation
Thermal treatments such as pasteurization are mostly considered useful for bacterial inactivation, but undesirable effects such as unwanted flavours and nutrient losses have encouraged research on novel processing techniques. Ultrasound treatment is applied as a processing aid to inactivate microbes. Various textural and physiological changes, i.e., the thinning of cell membranes and the production of free radicals, are the main mechanisms by which microorganism inactivation takes place 
. Transient cavitation will produce localized hot spots up to 4500–5000 K, and pressures > 199 MPa produce shock waves and free radicals, whereas stable cavitation will produce micro streaming accompanied by high shear 
. All these contribute to damage of the cell wall and membrane, resulting in cell death. It was reported that ruptured and disintegrated cells cannot be reviewed, which is advantageous over some other techniques in which damaged cells can recover if they encounter the right environmental conditions (temperature, pH, water activity, and nutrients) 
The resistance offered by five groups of microorganisms to the ultrasonic inactivation is in the order of spores > fungi > yeast > gram-positive cells > gram-negative cells. The resistance of viruses to ultrasound is high, but not enough data is yet available to compare it with the other microorganisms. Microbial destruction can also be accomplished by combining ultrasound with other treatments, such as heat (thermosonication), low static pressure (monosonication), ultraviolet light, or antimicrobials. Sonication combined with high pressures and temperatures (manothermosonication at 400 kPa/59 °C) was applied by Lee et al. 
for the control of Escherichia coli (E. coli) K12
populations in apple cider, and they achieved a 5-log reduction in 1.4 min and in 3.7 min when sonication was combined with a lethal temperature—whereas treatment via sonication alone takes 15.9 min for the same E. Coli
reduction. A sonication of yeast cells in Saccharomyces cerevisiae
2200 strain using a 20-kHz horn-type sonicator was carried out by Fabiszewska 
. After sonication, the count of live yeast cells decreases by 100 to 1000 times, compared to their initial count, expressed as Colony Forming Units CFU/cm3
. The efficacy of microbial destruction is governed by amplitude and frequency of the ultrasonic waves, the exposure/contact time, the composition and volume of the food to be processed, and the conditions 
. It was observed that the number of bubbles undergoing cavitation per unit of time increases at higher amplitudes, which resulted in a higher inactivation rate of the microorganisms. The majority of the research has been conducted to determine the effect of ultrasound on the microbial inactivation of fruits and vegetables 
. The inactivation of microorganisms using heat treatments and ultrasound and their D values (time for 90% reduction of microorganisms) are summarized in Table 1
Table 1. D values of microorganisms inactivated by using heat, ultrasound, thermosonication, monosonication, and manothermosonication.
||D Value (min)
||3.1(20 kHz, 124 μm)
||1.9 (1 min US, 55 °C) 0.73 (1 min US, 60 °C)
||Escherichia coli K12
||0.44 (100 kPa)
||0.40 (300 kPa)/0.27 (MTS 300 kPa 61 °C)
||0.28 (MTS-20 kHz, 117 μm, 200 kPa, 56 °C)
||5.06 (120 μm) 4.94 (120 μm, 500 ppm vanillin (Vi)) 1.09 (120 μm, 500 ppm potassium sorbate (KS))
||1.20 (120 μm) < 0.5 (120 μm, 500 ppm Vi) < 0.5 (120 μm, 500 ppm KS)
||9.59 (120 μm) 8.57 (120 μm, 500 ppm Vi) 7.15 (120 μm, 500 ppm KS)
||5.33 (120 μm) 6.47 (120 μm, 500 ppm Vi) 4.19 (120 μm, 500 ppm KS)
||10 (400 W, 24 kHz, 120 μm)
||Mesophilic aerobic, Lactic acid bacteria, Coliform bacteria, yeast
||750 W, 20 kHz, 6.8–126 μm
||Salmonella Typhimurium, Escherichia coli
||80 and 37 kHz, (330 W), pulsed modes, frequency amplitude (40% and 100%)
3.2. Ultrasonic Cutting
Because of the increasing demand for an improved quality of food-cutting processes with high accuracy, excellent cut faces, reduced smearing, low product loss, less deformation, less tendency to shatter for brittle products, and the ability to handle sticky or brittle foods, ultrasonic cutting is becoming increasingly important 
. Ultrasonic cutting is the size-reduction operation that utilizes the vibrating energy of the ultrasound, which superposes with the conventional blade movement, to improve the cutting efficiency and quality of the product 
. An ultrasonic cutting machine is composed of an ultrasonic transducer, a power supply unit, a horn, and a cutting knife. In the ultrasonic cutting mechanism, due to the high-frequency vibrations of the cutting blades, the food and cutter experience alternative contact and separation at a high deformation rate, though smaller deformations result in a reduced total cutting force and an avoidance of transversal cracks and crumbling, with a reduction of cutting-surface roughness 
The piezo-electric transducer converts the voltage that is supplied by a generator into a corresponding mechanical displacement at the outlet of the piezo element. The amplifier then transmits this displacement to the sonotrode, which induces an oscillation of a defined frequency at the cutting edge of the sonotrode 
. The cutting mechanism is more effectively used for cutting viscoelastic and viscoplastic foods, fragile and frozen foods, as well as heterogeneous products. The quality of food cut by ultrasonics, and the cutting efficiency, depends on the geometry of the cutter, the direction of the vibration of the cutter relative to the cutting direction, the frequency and amplitude of the cutter, and the product’s properties, such as its microstructure, moisture or fat content, or temperature sensitivity 
Ultrasonic cutting technology has been predominantly applied for cheese, bakery and confectionery products, and foods with smooth textures. Ultrasonically cut food showed a shining and smooth surface appearance of samples as compared to control 
. The technique is most suitable for porous products that possess high compressibility, high elasticity, low adhesiveness, and have a low content of lubricant liquid, which leads to frictionless cutting and deformation forces along the flanks of the cutting blade 
. Zahn et al. 
studied the relationship between cutting force requirement and magnitude and cutting velocity for baked materials (white bread, malted bread, sunflower grain bread, hamburger buns, milk rolls, and pound cakes) 
. It was observed that cutting work (Wc) decreases with an increase in vibration speed. However, a combination of the highest cutting velocity and the maximum vibration speed resulted in a significant reduction of cutting work for white bread. An inverse relationship exists between the average cutting force and the contact time between the product and the cutting device. For various foods, an ultrasonic frequency or amplitude (or both) is inversely proportional to cutting forces, whereas, irrespective of ultrasonic assistance, linear cutting velocity is directly proportional to the resulting forces. Additionally, as the cutting depth increases, the power consumption of the cutting system increases the amount of work required for ultrasound cutting (WUS), which is directly proportional to protein content and inversely proportional to the fat dry matter and moisture to the solid-non-fat ratio. Thus, reduced product deformation, higher cutting quality, and lower amount of product waste make the ultrasound cutting technique more impressive 
3.3. Ultrasonic Drying
Drying of the food materials is the most common and promising method for stabilizing the product. Traditional drying techniques cause adverse effects, such as shrinkage, discolouration, and the oxidation of vitamins. Additionally, rising energy costs, increasing quality requirements at reduced nutritional losses, and adverse environmental impacts have resulted in an increased interest in the development of modern food-drying technologies 
. The use of ultrasound accelerates the drying process without causing a severe temperature change. It has been suggested that mass transport kinetics can be increased by using high-intensity ultrasound. Moreover, ultrasound can decrease the boundary layer thus resulting in a decrease in resistance to mass transfer required for the drying process 
. An experimental study was carried out by Ortuno et al. 
on the convective drying kinetics of orange peel slabs at 40 °C and 1 m/s with and without power ultrasound, and they interpreted that oscillating velocities, pressure variations, and microstreaming created by ultrasound not only reduced the boundary layer thickness but also increased the water transfer to the air phase. In another study, ultrasound-assisted convective drying of apple shortened the drying time to 160 min, which incurred purely under convective drying 
, whereas combining the treatments of convective drying (50 °C), microwave (100 W), and ultrasound (200 W) reduced the drying time by 79%, as compared to convective drying alone for raspberries 
. An ultrasonic pretreatment on Andean blackberry was applied before convective drying by Romero and Yépez 
, who found greater antioxidant retention than the control due to a reduced processing time and lower drying temperatures. Recently, the application of ultrasound in food-drying methods has gained popularity among researchers for the drying of hydrophilic and lipophilic nutrients for microencapsulation 
, the drying of deformable porous materials 
, and ultrasound-assisted infrared drying of jackfruit 
. The different techniques of ultrasound-assisted drying of various fruit and vegetable crops have been explained in Table 2
Table 2. Effect of ultrasound during drying.
||Ultrasound Processing Parameters
|Ultrasound-assisted osmotic drying
||Indirect sonication: 25 kHz, 1.75 kW Osmotic solution: 70° Brix Immersion time: 60 min Air drying: 70 °C
||Initial moisture content: 91.3 ± 0.6% wet basis (w.b.) Final moisture content: 19.5 ± 3 (w.b.) Total dehydration time: 300 min. Drying time reduced by 33%.
|Ultrasound-assisted convective drying
||Ultrasound: 21.8 kHz, 60 W Air drying: 70 °C
||Initial moisture content: 90.5 ± 0.27 (w.b.) Final moisture content: 23.07% (w.b.). Total drying time: 2.2 h. Drying time reduced by 44%.
|Ultrasound-assisted osmotic dehydration
||Ultrasound: 25 kHz, 700 W
||It was proved that intermittent drying of cherries preceded by ultrasonic-assisted osmotic dehydration contributes to shorter drying time, better colour preservation, and smaller water activity.
|Ultrasound-assisted convective drying
||Ultrasound: 21.8 kHz, 30.8 W Air drying: 70 °C
||Passion fruit peel
||Initial moisture content: 87.5± 1.9 (w.b.) Final moisture content: 32% (w.b.) Total drying time: 3.9 ± 0.7 h.
|Ultrasound-assisted radiation drying
||Ultrasound: 1200 W, 20 kHz Sonication time: 5 s Drying: 62 °C
||Final moisture content: 10 ± 0.5% (d.b.) Drying time increased with increasing ultrasonic power levels.
|Ultrasound-assisted vacuum drying
||Sonication time: 10 s Drying: 65–75 °C
||Final moisture content: 12–13% dry basis (d.b) Drying time was decreased by 53%.
||1000 W and 50 ℃
||Decrease of 0.65-fold in adhesiveness values. Population of free water increased from 2.71% to 11.35%. Decreased the content of rancid and sour compounds. Accelerated the formation of esters.
|Ultrasound-assisted microwave dryer
||28 kHz, 70 W, 30 min
||Reduction in drying time by 63%.
Least-specific energy consumption: 23.75 ±2.22 MJ/kg Lowest shrinkage: 31.8 ±1.1%